The present invention relates to rigid monocular borescopes and endoscopes which are well known devices for viewing objects at remote or inaccessible locations. The invention in particular relates to means for providing an accurate direction of view in such rigid borescopes and endoscopes.
Although there are many detailed design variations, rigid borescopes and endoscopes generally consist of the basic arrangement shown schematically in FIG. 1. When an object O lies within the scope""s field of view (f.o.v.), an image I of the object O is formed within the insertion tube 12 of the scope 10 by means of an objective lens assembly 14. This intermediate image I is then transferred or relayed to the proximal end of the scope 10 by a series of relay lenses 16 to form a final intermediate image superimposed on a field mask 18. The purpose of the field mask 18 is to provide a sharp, well-defined edge to the specified field of view. Finally an ocular lens system 20 then projects this final image to form a virtual image some distance away, say about 1 m, so as to present a comfortable view to the eye E (a normal, unaided eye can generally focus comfortably on objects from a distance of about 25 cm to infinity). Additionally an adaptor lens attachment (not shown) may be fitted to re-focus this image on to a camera or CCD chip to provide an image of appropriate size for the camera.
The distal end of the scope 10 will generally use a prism arrangement 22 to establish the required direction of view (d.o.v.), unless this is a 0xc2x0 forward viewer in which case no prism is required. The direction of view is the angle between the longitudinal axis L of the scope 10 and the viewing axis V where the viewing axis is defined as the line bisecting the extreme directions delineating the diametric edges of the visible field of view. The actual direction of view that is achieved in practice may vary from the design specification for a number of reasons:
the prism 22 may not be set at precisely the correct angle
the individual lenses 14, 16 comprising the objective and relay systems may not be manufactured such that the optical and mechanical centres coincide precisely
the individual lenses 14, 16 comprising the objective and relay systems may not be precisely centred or aligned in the lens tube 12
In a borescope or endoscope there may be large numbers of individual lenses and the build up of very slight centring errors of this type may easily result in a shift in the measured d.o.v. outside acceptable limits. The conventional method of controlling this is to place very tight tolerances on the items listed above in order to ensure the resultant d.o.v. is within the specified limits (typically xc2x15xc2x0). For scopes where the specified tolerance for d.o.v. is particularly tight (e.g. xe2x89xa63xc2x0) it may not be possible (or would be prohibitively expensive) to control these individual tolerances to the required level of precision. There is therefore a need to achieve a high tolerance d.o.v. without relying on particularly tight individual centration tolerances.
Accordingly, the present invention provides apparatus for use as a monocular borescope or monocular endoscope, comprising a tube having a distal end and a proximal end connected to a housing, a viewing port adjacent a distal end through which an object may be viewed in use, an image relaying means operable to relay an image of the object to a viewing means provided in the housing, the viewing means including an ocular lens and a mask positioned distally of the ocular lens and having an aperture through which the image is viewed, wherein the image relaying means defines a longitudinal axis and the aperture of the mask defines a central axis and wherein the central axis is parallel to and offset from the longitudinal axis.
In a first embodiment, the ocular lens defines a central axis which is coincident with the longitudinal axis.
In a second embodiment, the ocular lens defines a central axis which is coincident with the central axis of the aperture in the mask.
In order to provide the desired offset, the mask may be mounted so as to be adjustable into the correct position. Alternatively, the mask may be moved and replaced with a second mask having the correct offset. However, the current preference is for the mask to be mountable in different rotational orientations each providing a different offset between the longitudinal axis of the image relaying means and the central axis of the mask. Preferably the ocular lens is adjustable together with the mask. This allows the mask to be provided with different offsets, but does not require a selection of different masks to achieve this aim.
The present invention also provides a method of providing a desired direction of view in a monocular borescope or monocular endoscope, the borescope or endoscope comprising a tube having a distal end and a proximal end connected to a housing, a viewing port adjacent to the distal end through which an object may be viewed in use, an image relaying means operable to relay an image of the object to a viewing means provided in the housing, and the viewing means comprising an ocular lens; the method comprising the steps of providing a mask distally of the ocular lens, the mask having an aperture through which the image is viewed and the aperture having a central axis; determining the angular difference between the actual direction of view and the desired direction of view; calculating the amount and direction of offset of the central axis of the mask aperture relative to the longitudinal axis required to achieve the desired direction of view and detaching the mask and fitting a mask having an aperture defining a central axis with the required offset.
The steps of detaching and fitting the mask preferably comprise detaching the mask, rotating the mask to a different rotational orientation with the required offset and fitting the mask into position. Preferably, the ocular lens is rotated together with the mask to the required offset.
The method may further comprise step of replacing the ocular lens with a second lens having a central axis coincident with the central axis of the second mask.
In the method, the amount of offset required may be calculated in accordance with the relationship:   δ  =            α      ·      η        ϕ  
where xcex4 is the amount of offset required, xcex1 is the angular difference between the actual direction of view and the desired direction of view, xcex7 is the size of the image at the position of the mask and xcfx86 is the field of view of the scope.